1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method which control an electrostatic latent image to be formed on a photosensitive member.
2. Description of the Related Art
In recent years, the resolution of electrophotographic image forming apparatuses has increased to 400 pdi (63.5-μm dot pitch), 600 pdi (42.3-μm dot pitch), and 1200 pdi (21.1-μm dot pitch), and most recently, image forming apparatuses offering a high resolution of 2400 pdi (10.6-μm dot pitch), which was offered in the past by printing machines, have emerged.
With the increase in resolution, there has been a growing demand for image forming apparatuses which can write an image on a photosensitive member with high positional accuracy.
For example, at a low resolution of 600 pdi, the dot pitch width (the size of one dot) is as wide (large) as 42.3 μm. Here, assume that a variation in the rotational speed of the photosensitive member (variation in the rotational speed), a variation in the scanning pitch of a laser scanner (variation in the intervals of scanning lines in the sub scanning direction), or a displacement (shift) of an image caused by a change in the relative mounting positions of the photosensitive member and the laser scanner is 10 μm. In this case, the percentage of the displacement relative to the dot pitch width 42.3-μm is 25%, and unevenness in an image is inconspicuous and within an allowance. At a high resolution of 2400 pdi, however, the dot pitch width (the size of one dot) is 10.6 μm. Thus, if an image is displaced 10 μm, the percentage of the displacement relative to the dot pitch width is approximately 100%, and an image is displaced by one pixel or one scanning line to cause an overlap. Thus, there is the problem that a great unevenness of an image occurs due to the impossibility to ensure image uniformity as well as displacement of thin-lined edges.
On the other hand, it has been conventionally thought that an even image can be formed by detecting a displacement as described above and suitably controlling the apparatus according to the detected variation.
For example, there has been proposed an image forming apparatus in which an encoder provided on a rotary shaft of a photosensitive member detects the rotational angular velocity of the photosensitive member, and the rotational speed or the like of the photosensitive member is controlled in accordance with the detected angular velocity (see e.g. Japanese Patent No. 3259440).
However, with the above conventional image forming apparatus in which the encoder provided on the rotary shaft of the photosensitive member detects the rotational angular velocity of the photosensitive member, only the angular velocity of the photosensitive member can be detected, and the moving speed of the surface of the photosensitive member on which attention should be originally focused cannot be detected. Specifically, in a case where the surface of the photosensitive member deflects from the rotation axis of the photosensitive member or in a case where the surface of the photosensitive member itself is distorted, it is difficult to detect a variation in the moving speed of the surface of the photosensitive member. For this reason, with the conventional image forming apparatus, it is impossible to prevent image displacement caused by a variation in the moving speed of the surface of the photosensitive member even if the rotational speed or the like of the photosensitive member is controlled in accordance with the angular velocity. Therefore, it is impossible to form an even image while maintaining high resolution.